1. Field of Invention
The present invention relates to an amplifying circuit and system, and more particularly to a power amplifying circuit and system, which is able to realize functions of a CLASS AB amplifier and a CLASS D amplifier.
2. Description of Related Arts
CLASS AB amplifiers and CLASS D amplifiers are two kinds of power amplifiers which are widely applied in the field of audio frequency in the conventional technology, and are both used to drive sound equipments, such as speakers, headphones, and earphones.
The CLASS AB amplifiers belong to linear amplifiers, and because the CLASS AB amplifiers have simple application, do not need overmuch external device, and have no problem of EMI (Electromagnetic Interference), the CLASS AB amplifiers are widely applied on various handheld devices. However, the CLASS AB amplifiers have low work efficiency. In an usual application, the work efficiency of the CLASS AB amplifier is only about 30%, so a majority of energy is consumed in an interior of a chip as being converted into heat, and the CLASS AB amplifier having high power usually requires for an extra cooling equipment. In the conventional technology, the CLASS AB amplifier drives a FET (field-effect tube) by an operational amplifier, to further drive the speaker.
The CLASS D amplifiers belong to switch-typed amplifiers. Because of the problem of EMI, application of the CLASS D amplifier is limited. However, the CLASS D amplifier has an advantage of high efficiency, and in the usual application, the efficiency of the CLASS D amplifier is more than 80%. In the conventional technology, the CLASS D amplifier converts an analog signal into a PWM/PDM (pulse width modulation/pulse density modulation) signal having 0 and 1 by a PWM/PDM generator, and a driver amplifies a driving ability of the PWM/PDM signal to drive the FET, and to further drive the speaker.
In order to not only play music for a long time but also receive FM signal without disturbing of EMI in application of handheld devices, such as MP3s, it is necessary to provide a power amplifying circuit and system able to realize composite functions of CLASS AB amplifier and CLASS D amplifier.